A well-known thermal fixing device includes an endless fusing film, a heater disposed in an internal space of the fusing film, a pressure roller, and a heating plate (nip member) defining a nip region relative to the pressure roller through the fusing film. The guide members are disposed respectively on both widthwise ends of the fusing film. While a recording sheet is conveyed between the fusing film (nip member) and the pressure roller, a developing agent image formed on the recording sheet is thermally fixed.